Here's the main text linked-to from Tom's answer, in case that page eventually changes or goes away:
Hooks do not propagate
In Mercurial, hooks are not revision controlled, and do not propagate
when you clone, or pull from, a repository. The reason for this is
simple: a hook is a completely arbitrary piece of executable code. It
runs under your user identity, with your privilege level, on your
machine.
It would be extremely reckless for any distributed revision control
system to implement revision-controlled hooks, as this would offer an
easily exploitable way to subvert the accounts of users of the
revision control system.
Since Mercurial does not propagate hooks, if you are collaborating
with other people on a common project, you should not assume that they
are using the same Mercurial hooks as you are, or that theirs are
correctly configured. You should document the hooks you expect people
to use.
In a corporate intranet, this is somewhat easier to control, as you
can for example provide a “standard” installation of Mercurial on an
NFS filesystem, and use a site-wide ~/.hgrc file to define hooks that
all users will see. However, this too has its limits...
That last warning may have been referring to the very next section which adds:
If you deploy a system- or site-wide ~/.hgrc file that defines some
hooks, you should thus understand that your users can disable or
override those hooks.